build: update sphinx tool

Use the improved version from the ndn-cxx repository

Refs: #5298
Change-Id: I9343a1bf7b136e6690ae39c7f6f21b7bf43343b9
diff --git a/wscript b/wscript
index acd4008..945da93 100644
--- a/wscript
+++ b/wscript
@@ -12,7 +12,7 @@
     opt.load(['compiler_cxx', 'gnu_dirs'])
     opt.load(['default-compiler-flags',
               'coverage', 'sanitizers', 'boost',
-              'doxygen', 'sphinx_build'],
+              'doxygen', 'sphinx'],
              tooldir=['.waf-tools'])
 
     optgrp = opt.add_option_group('NAC Options')
@@ -26,7 +26,7 @@
 def configure(conf):
     conf.load(['compiler_cxx', 'gnu_dirs',
                'default-compiler-flags', 'boost',
-               'doxygen', 'sphinx_build'])
+               'doxygen', 'sphinx'])
 
     conf.env.WITH_EXAMPLES = conf.options.with_examples
     conf.env.WITH_TESTS = conf.options.with_tests